1. Field of the Invention
The instant invention is directed to a process for isomerizing an allylic ether. More specifically, the present invention is directed to a process wherein an allylic ether is isomerized such that the allylic ether functional group, remote from the terminal end of the compound, is isomerized so that it becomes the terminal functional group.
2. Background of the Prior Art
Allylic ethers are well known reactive compounds wherein the functional allylic ether group reacts with a whole host of reactive groups. Those skilled in the art are aware of the greater utility of allylic ether compounds containing an allylic ether group on the terminal carbon atom compared to those compounds where the allylic ether group is on a non-terminal carbon atom. This increased utility of allylic ether compounds possessing terminal reactive allylic ether groups is due to the use of such compounds as intermediates in the synthesis of commercial products.
Although allylic ether compounds having terminal functional allylic ether groups are known in the art to possess far more commercial importance than allylic ether compounds whose allylic ether groups are substituted on a non-terminal carbon atom, oftentimes synthesis routes which result in the formation of an allylic ether compound effect such less useful compounds in which the functional allylic ether group is positioned on a non-terminal carbon atom.
An important example of the above remarks is provided by the synthesis of azelaic acid, a commercially important product. Azelaic acid can be synthesized from an allylic ether-containing octadiene in which the allylic ether group is bonded to the terminal carbon atom of the octadiene chain. The usual synthesis of an alkoxy-containing octadiene allylic ether starts with butadiene. The reaction of butadiene produces an alkoxy-containing octadiene wherein the alkoxy group, that is, the allylic ether group, is positioned on a non-terminal carbon atom. Thus, an isomerization process to transfer the allylic ether group from a non-terminal carbon atom of the octadiene chain to a terminal carbon atom on that chain would, in itself, establish an important advance in the art.
Allylic ether group rearrangement of allylic ether compounds are very uncommon in the art. Although isomerization of allylic ester compounds is known, there is little in the art directed to the rearrangement of allylic ethers compounds. Takahashi et al., Bull. Chem. Soc. Japan 45, 230 (1972) presents the closest known art relating to a process for rearranging allylic ethers wherein an allylic ether group is moved to the terminal carbon from another position on the compound chain. In Takahashi et al. a catalyst comprising a mixture of dichlorobis-(triphenylphosphine)palladium (II) and sodium phenoxide is used to catalytically isomerize octadienes having allylic ether functionality. As suggested by the catalyst used, the teaching of Takahashi et al. is limited to the utilization of a Group VIII metal catalyst, specifically, palladium. It is, furthermore, noted that in the case where the allylic ether group is aliphatic, the yields obtained are very low.
U.S. Pat. No. 2,429,411 discloses a process for synthesizing an unsaturated ether which may be allylic. However, the starting reactant is not an allylic ether but rather a dienol. The process involves the use of a mineral acid catalyst. Such a process is far removed from the isomerization of an allylic ether to produce an isomer allylic ether compound possessed of terminal functionality in the presence of a metal-containing catalyst.
U.S. Pat. No. 3,755,450 to Kurtz et al. teaches a method for isomerizing octyl compounds by catalytic conversion of the octyl compound in the presence of a palladium catalyst complex with triphenylphosphine. The '451 patent is directed to allylic alkenyl ethers, including allyl octadienyl ether. It is emphasized that this specific reaction is one of literally hundreds of possibilities within the scope of the teaching of this patent. It is furthermore emphasized that no teaching, disclosure or suggestion is made of utilizing a metal-containing catalyst system which includes copper.
The above remarks establish the need in the art for a new process for isomerizing allylic ethers such that the allylic ether group is moved from a non-terminal to a terminal carbon atom. The above discussion, furthermore, establishes not only the need in the art of a commercial process for conducting such a transformation but, moreover, the requirement for a process in which such a process is conducted without the necessity of using very expensive palladium-containing catalysts.